Generally, combustion turbines have three main assemblies, including a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor assembly compresses ambient air. The compressed air is channeled into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is typically at a temperature of between 2500 to 2900° F. (1371 to 1593° C.), and is expanded through the turbine assembly. The turbine assembly generally includes a rotating assembly comprising a centrally located rotating shaft and a plurality of rows of rotating blades attached thereto. A plurality of stationary vane assemblies, each including a plurality of stationary vanes, are connected to a casing of the turbine and are located interposed between the rows of rotating blades. The expansion of the working gas through the rows of rotating blades and stationary vanes or airfoils in the turbine assembly results in a transfer of energy from the working gas to the rotating assembly, causing rotation of the shaft.
The vane assemblies may typically include an outer platform element or shroud segment connected to one end of an airfoil for attachment to the turbine casing and an inner platform element connected to an opposite end of the airfoil for attachment to the compressor diffuser exit structure. The outer platform elements may be located adjacent to each other to define an outer shroud, and the inner platform elements may be located adjacent to each other to define an inner shroud. The outer and inner shrouds define a flow channel therebetween for passage of the hot gases past the stationary airfoils.
The first row of vane assemblies, which typically precedes the first row of rotating blades in the turbine assembly, is subject to the highest temperatures of the working gas, and the support scheme for the first row vanes must provide a fail-safe support structure under an extreme of structural and thermal loading. Typically, the first row vanes have been “simply” supported, where the outer platform elements of the first row vanes are attached to the turbine structure, i.e., to an inner turbine casing, and the inner platform elements are attached to the compressor exit diffuser structure. During transient and steady state operation of the turbine, the axial displacement of the inner and outer support structures is not the same due to differential thermal growth of the two structures. This produces significant differential axial displacements between the inner and outer platform elements of the vane. The differential axial displacements can produce high stresses within the vane. In addition, the differential axial displacements can cause ID-to-OD rocking of the vane between the inner platform element of the vane and the transition duct from the combustor, potentially resulting in substantial gas leakage and loss of efficiency due to the large relative displacement.
One approach to solving the problems associated with the differential thermal displacement is to support the vane entirely at the OD of the turbine, referred to as a cantilevered vane. However, this approach can produce unacceptable stresses in the vane, particularly in heavily loaded vanes of more advanced turbine designs.
Accordingly, it is an object of the present invention to provide support at the vane OD and ID with this support being provided substantially by the OD of the turbine vane carrier, hence greatly reducing the vane rocking associated with transient and steady state differential thermal growth of the turbine.